1. Field of the Invention
The present invention relates to a radio communication apparatus and radio communication method in a CDMA/TDD radio communication system in which multiple accesses are performed by a spread spectrum communication where information is spread with a spreading code to transmit in a spread frequency band and communications in the same radio frequency are performed alternately via a reverse link and a forward link in time division.
2. Description of the Related Art
Conventionally, as a radio communication system using a CDMA (Code Division Multiple Access) system, US Standard IS-95 is known. As a duplex system in IS-95, FDD (Frequency Division Duplex) is used. As the duplex system, TDD (Time Division Duplex) is also known. In the TDD system, transmissions and receptions are performed in the same frequency band, called a Ping-Pong system, where communications in the same radio frequency are performed alternately via a reverse link and a forward link in time division.
A multiple access system is a link connection system where a plurality of stations perform communications concurrently in the same frequency band. The CDMA technique is used to perform multiple connections by spread spectrum communications where an information signal is spread with a spreading code to transmit in a spread frequency band.
A direct sequence spread spectrum system is a system where an information signal is multiplied by a spreading code in the spreading. In a direct sequence CDMA, a plurality of communication links share the same frequency, thereby resulting in a problem (near-far problem) that it is necessary to acquire the same communication level at each reception side, which should be solved to achieve a CDMA transmission system.
The near-far problem is severer in receptions of a base station for receiving radio waves concurrently from a plurality of mobile stations located at different distances from the base station. Therefore, it is mandatory at mobile station sides to perform a transmission power control corresponding to a state of each transmission path.
FIG. 1 is a schematic diagram illustrating base stations and a mobile station in the base stations controlled communication areas. FIG. 1 illustrates a case where three base stations are present.
In FIG. 1, the communication area of base station 1 is cell 5, the communication area of base station 2 is cell 6, and the communication area of base station 3 is cell 7. The scales and forms of communication areas vary depending on propagation environments.
Mobile station 4 acquires synchronization of spreading codes transmitted from base stations 1 to 3 when turned on. First, mobile station 4 extracts signals of base stations 1 to 3 from signals including signals of base stations 1 to 3, and starts the synchronization acquisition of the spreading codes.
The initial synchronization acquisition is explained with reference to FIG. 2. FIG. 2 is a timing diagram of each cell frame and a long spreading code. In IS-95, base stations 1 to 3 are synchronous to each other and the same basic timing is applied in transmission frames of each of cells 5 to 7. As spreading codes, a short spreading code and a long spreading code are multiplexed to be used. The short spreading code is, for example, 64 chips, and the long spreading code is, for example, 40,960 chips.
In a forward link, a sort of long spreading code is common and one in the system. Each of cells 5 to 7 uses the sort of long spreading code by shifting a phase (differing a timing of a head of the code). Generally, the timings of basic frames k, k+1 and k+2 of cells 5 to 7 are not conformed to the timing of the long spreading code. Accordingly, each cell is identified by the phase shift variation (difference).
The system includes some physical channels, where timings of communication channel frames (communication frames) and timings of control channel frames (control frames) except SYNC channel frames (synchronism frames) are conformed to the basic frames k, k+1 and k+2.
Only the timings of SYNC channel frames (SYNC frames) k′, k′+1 and k′+2 are not conformed to the basic frames k, k+1 and k+2 but conformed to the timing of the long spreading code.
A sort of short spreading code used in the SYNC channel is common and one in the system. Mobile station 4 detects the correlation of a received signal with a spreading code in which the long spreading code and the short spreading code of the SYNC channel are multiplied. The correlation detection is performed by varying the timing gradually until the correlation level exceeds the threshold value.
In the above manner, mobile station 4 detects the timing of the long spreading code from either of cells 5 to 7. Since the timings of SYNC channel frames k′, k′+1 and k′+2 are conformed to the timing of the long spreading code, it is possible to demodulate and decode SYNC channel signals according to the timing of the long spreading code.
The SYNC channel informs a difference of timings between basic frames k, k+1, k+2 and the long spreading code of the current cell (cell 5 in the configuration in FIG. 1) and a difference of timings between basic frames k, k+1, k+2 and the long spreading code of the neighboring cell (cell 6 or 7 in the configuration in FIG. 1). Accordingly, mobile station 4 is capable of acquiring the timing of basic frames k, k+1 and k+2, thereby allows mobile station 4 to demodulate and decode different control channel signal.
Since the SYNC channel also informs the difference of timings between the long spreading code of the neighboring cell and basic frames, mobile station 4 is capable of demodulating and decoding different control channels signals of neighboring cell 6 or 7 and of comparing a reception level of cell 5 which SYNC channel is already detected with a reception level of neighboring cell 6 or 7. Then mobile station 4 selects a control channel of another cell with higher reception level to receive.
However, in the conventional initial synchronization method described above, when it is assumed that the long spreading code length is, for example, 32,768 chips (26.667 [ms]), it is necessary to repeat the correlation detection with respect to 32,768 timings (more than if oversampling is considered). In this case, it takes 6.82 [s], as shown below, to repeat detecting the correlation in short spreading code length (for example, 256 chips) 32,678 times (1/1.2288 [MHz])×(256/2)×32768=6.82 [s]. In the case of calculating an average, it takes a half of the time, which is 3˜4 [s]. Actually, the processing time including call connection processing is regulated maximum 15 [s], which suggests that it takes relatively a long time to detect the correlation of the long spreading code.
In addition, the transmission power of the SYNC channel is a half that of the communication channel. When it is assumed that the number of concurrently connected communication channels in a cell is, for example 20, one-fortieth of the frequency utilization efficiency is decreased. Accordingly, the frequency utilization efficiency is decreased due to the SYNC channel.
In FDD, frequency bands between a reverse link and a forward link are different and communications in each link are continuously performed. On the contrary, in TDD, communications in the same frequency band are alternately performed in a reverse link and a forward link. In other words, in the case of receiving forward link signals in mobile stations, an area where a signal to be received is present and an area where a signal to be received is not present are switched. Before a mobile station acquires the synchronization with a base station, a switching timing of the reverse link and the forward link is unknown to the mobile station. In other words, in TDD, a mobile station has not acquired the switching timing of the reverse and forward link.
Further, in the case where the switching timing of the reverse link and forward link is synchronized with the frame timing, the timings of the long spreading code are limited by some of switching timings of the reverse and forward link in the frames. In other words, in TDD, in the case of synchronizing the switching timing of the reverse and forward link with the frame timing, the long spreading code timings are limited. Such limitation brings a difficult planning of base station locations.